1. Field of the Invention
This invention relates generally to transceivers. More particularly, the invention relates to direct current correction in multi-mode transceiver systems.
2. Related Art
Trends in portable transceivers, such as cellular handsets, have progressed towards smaller form factors, high data rate capabilities, as well as a variety of features such as integrated cameras, music download abilities, and/or global roaming. Accordingly, multi-mode systems such as GSM (Global System for Mobile Communications), CDMA 2000 (Code Division Multiple Access), WCDMA (wide-band CDMA), among others include radio architectures that have evolved towards direct-conversion receivers, which often consume less board space and typically cost less than other types of receivers.
The classical challenges in the design of direct conversion receiver architectures are well-known and include issues centering around direct current (DC) offset. DC offset typically results from a signal applied at two inputs (differential input), and represents the difference in the common mode voltage between the two inputs. In direct conversion receivers, the signal from an associated antenna is directly down-converted to baseband (e.g., DC) and then amplified before further signal processing, thus providing significant gain at baseband. Due to this large gain, any DC offsets generated in the front-end of the receiver (e.g., due to local oscillator (LO) leakage, non-linearities, etc.) can potentially saturate the baseband section and hence corrupt the desired signal, which is typically much smaller in signal level than the unwanted DC signal. Several methods of DC offset cancellation have been developed to minimize DC offset in many types of receivers, including direct conversion receivers, yet these methods may cause significant degradation in signal quality and/or a significant reduction in portable transceiver standby times.
Therefore, it would be desirable to provide a portable transceiver that provides enhanced features without significant signal degradation and/or excessive power consumption.